1. Field of the Invention
The present invention pertains to a flip-chip semiconductor die assembly and, more particularly, to micro-size bumps and recessed contacts for self-aligned contact of the die to a substrate, and specifically to methods for forming the bumps and mating contacts.
2. State of the Art
As the complexity of integrated circuits on semiconductor dice has increased, semiconductor die manufacturers and assemblers have found a correspondingly increased need for improved input and output connections. A flip-chip arrangement is one conventional arrangement used to take advantage of its potentially higher number of arrayed input and output connections; that is, more such connections can be placed on the active surface of a die than with wire-bonding, TAB, or other conventional connection techniques. In a conventional wire bonded arrangement, the number of connections that can exist in a given surface area of a die is limited because of the diameter of the wire ball to be formed from the bond wire at the connection point or bond pad on the die surface, as well as by the number of wires which can be extended from bond pads to a lead frame or carrier substrate without shorting. The pitch, or nominal distance between the center of any two connection points, is generally limited to approximately 0.1 mm, although some arrangements have achieved a pitch of as low as 0.08 mm. Simply put, the wire balls are too bulky to allow a more dense array of connections, and potential mutual interference by the wires limits the usable patterns of such connections.
By contrast, a conventional solder-bumped flip-chip arrangement allows a high density of connections per given area of active surface on the die, and the least amount of die-to-carrier connection time because of the ability to effect all connections simultaneously. In a conventional flip-chip arrangement, solder bumps are formed or deposited on a semiconductor die, and the die ("chip") is turned over, i.e. flipped, and then aligned with mirror-image solder bumps or bond pads on another die, or terminal pads of a die carrier or a printed circuit board or other, similar carrier substrate. By reflowing the solder after contact of the bumps with the pads or cooperating bumps on the mating component of the assembly being fabricated, a simultaneous electrical, mechanical, and thermal connection of each cooperating pair of contact points is achieved. Since bump size can, with some techniques, be smaller than wire ball size and bumps can, in some instances, be placed more accurately than wire balls, the potential density of bumps can exceed that of wire bonds, reaching a corresponding pitch of as low as 0.01 mm. Nevertheless, bump pitch is limited by the selection of bump size, bump shape, and bond pad metallization characteristics. As detailed below, when an improper combination of these elements is selected, the bumps may spread outward too far and form unwanted connections to other bond pads during reflow of the solder.
One of the first solder-bumped flip-chip arrangements was created using so-called Controlled Collapse Chip Connection (C4) technology. The technology involves, first, laying down a passivation layer on the surface of a semiconductor die which covers the bond pads where connections will be made between the die and a substrate. Next, holes are formed in the passivation layer over the bond pads and one or more layers of metallization are typically deposited over the exposed bond pads. Finally, solder bumps (typically of a tin/lead alloy, although other alloys are sometimes employed) are deposited on the metallized areas and a preliminary reflow performed so that the bumps take on a semi-spherical shape. Later, after alignment with terminal pads of conductive traces of a substrate, a final reflow will form the permanent die-to-substrate electrical connections. The metallization deposited on each bond pad must be limited in circumference to the approximate size of the hole through which it contacts the bond pad. However, the metallization may extend up the walls of the hole in the passivation layer through which the bond pad is exposed, and onto the top surface of the passivation layer, although obviously avoiding contact with neighboring bond pad metallization.
One purpose of the metallization layer interconnecting the bump and the bond pad on the underlying active surface of the die is to provide improved solder adhesion to the bond pad. Another purpose is to control the contact area the bump will cover on the die surface by use of a very solder-wettable metal or alloy on the exposed surface of the metallization. The intent is to prevent the solder from spreading beyond the circumference of the deposited metallization. By controlling the contact area, the metallization partially controls the bump's height, since the bump will form a semi-sphere with a size somewhat dependent on the circumference of the metallized area on which it resides, as well as on the volume of the bump material. Understandably then, the metallized area is sometimes referred to as Ball-Limiting Metallurgy (BLM). If the volume of deposited solder becomes too large for a given contact area metallization, then the surface tension of the particular solder composition used will be insufficient to contain the molten solder in spherical form and the solder will overflow the metallization despite its presence. Even if the surface of the die is additionally coated with a low-surface tension material to inhibit spreading of the reflowed solder from the BLM, the effectiveness of such coatings is limited. The coating will probably help prevent incidental outflows from the BLM, unless a bump is too large and exceeds the surface tension of the molten solder. In that case, a low-surface tension coating will probably be insufficient to contain all of the escaping solder and avoid contact with another bond pad located nearby in a fine-pitch array. Thus, bump volume and pitch must be carefully considered and controlled to prevent defects in flip-chip connections.
The use of solder bumps to form connections between two dice, a die and a printed circuit board or other carrier substrate, or a carrier substrate and a higher-level package is well-known in the art. However, even though BLM is used on components of such assemblies carrying the solder bumps, the spacing or "pitch" of the bumps is limited by conventional technologies due to problems with preventing the bumps from flowing together during reflow of the solder. Many variations in the materials used in a C4 process and in the detailed process steps exist, since users have sought to match the technology to their particular applications, to meet reliability requirements, and to improve production efficiency and connection quality. The significant number of these variations is indicative of the complexity of conventional methods for forming solder bumps on dice and the number of problems inherent in the conventional methods. The complexity of forming adequate solder bump connections is further exemplified by the methods disclosed in U.S. Pat. Nos. 4,940,181; 5,477,086; 5,480,835; 5,492,235 and 5,505,367. The complexity of such methods contributes to the typical, relatively high cost of manufacturing solder-bumped dice, particularly as attempts are made to form smaller bumps with hopes to achieve a more densely-packed array of connections.
After forming bumps on a semiconductor die, the die must then typically be connected to another die, or to a printed circuit board or other carrier substrate. As indicated earlier, the die bumps are aligned with mirror-image terminal pads or solder bumps on the substrate to make the connection. Substrate bumps may generally be formed by the same methods used to form die bumps. However, the substrate bumps often possess a designed shape so as to facilitate aligning of the die bumps and making a reliable connection. In some instances, bumps of metals other than solder are employed, and connections are effected by means other than a reflow. In addition, metal-loaded polymer bumps have also been fabricated. See, for examples of the foregoing structures, U.S. Pat. Nos. 4,182,781; 5,246,880; 5,329,423; 5,346,857 (all non-solder metal bumps) and 5,508,228 (metallized compliant polymer bumps). Self-aligning connections may be desirable, and exemplary shapes previously designed for this approach are illustrated in U.S. Pat. Nos. 4,940,181; 5,019,673; 5,329,423 and 5,477,086. Absent designing a substrate bump or terminal pad with some sort of conforming shape wherein a projecting die bump can reside and, thus, self-align, highly accurate mechanical pre-reflow alignment of the die to the substrate must be achieved by another method. Thereafter, all the bumps on the die and cooperating contact areas on the substrate must be brought into relatively exact contact with one another and maintained in that position during both reflow and re-solidification of the solder. Because of the surface tension changes and capillary action that occurs during reflow and re-solidification of the solder bumps, maintaining a die in its proper position relative to a substrate may be more difficult than it first appears, particularly if some of the intended connection points are even slightly misaligned, which in turn tends to induce misalignment of other connection points due to the surface tension and capillary action of the solder material.
Given the added manufacturing cost of forming specially designed substrate bumps to match the shape of die bumps, some manufacturers have developed less costly self-alignment methods. For example, as disclosed in U.S. Pat. No. 3,811,186, spacers may be disposed between the die bumps such that the spacers nearly exactly occupy the gap between bumps. By using specifically-placed spacers of insulating material which may exceed the combined height of aligned die and substrate bumps, when the die and the substrate are brought into alignment and the assembly heated, the spacers will soften and reduce in height, permitting contact and fusion of the bumps in an aligned manner. When the assembly cools, the spacers will return to their normal height, elongating the fused, aligned bump connections. Attempts have also been made, as in U.S. Pat. No. 5,508,228, to eliminate the crucial need for self-alignment by using non-conductive adhesives surrounding the connection points to join metallized, compliant polymer bumps of a die to contact points on a substrate rather than relying on a solder bond. Additionally, bumps formed from conductive paste, as in U.S. Pat. No. 5,246,880, rather than solder, have been used to produce a higher aspect ratio bump than can be achieved with solder, and a bump of more precise and repeatable height. However, formation of a conductive paste bump according to the '880 process will generally take longer than formation of a solder bump, since the bump is "built" in a plurality of layering steps, and the paste curing time exceeds solder re-solidification time.
See also U.S. Pat. No. 5,445,994, assigned to the assignee of the present invention, for a disclosure of forming planar bond pad connectors by patterning a passivation layer with holes by using an overlying patterning layer of a dielectric material, etching holes down to bond pads on the die, filling the holes with a metal layer, and planarizing the metal layer to an endpoint within the patterning layer over the passivation layer.
Insofar as solder bumps become semi-spherical in shape when heated to a liquid state, an increase in the desired height of bumps will result in a decrease in the potential density of a bump array, as an increase in height of a semi-spherical bump necessarily results in an increased bump width. Taller, more slender bumps (the term "slender" indicating a bump height measurably more than the bump width, or an aspect ratio of height to width of greater than 1) of columnar or pillar configuration are desirable for two significant reasons. First, with relatively slender bumps for a given gap or clearance between the die and the substrate, more bumps can be disposed on the die without inadvertent lateral connection between bumps during solder reflow. Second, thermal expansion of the die and/or the substrate creates stresses which the solder bump connections will bear. A "fat" semi-spherical-shaped solder connection will not be able to endure as much flex and strain as a slender solder connection of equal height. Flex and strain capacity of solder connections becomes particularly important when a silicon semiconductor die has a significantly different coefficient of thermal expansion (CTE) than its corresponding carrier substrate. Under such circumstances, the substrate will typically have a larger CTE, and will thus expand and contract during heating and cooling cycles to a greater degree than the die, creating substantial stress in the solder connections.
To the inventors' knowledge, those of ordinary skill in the art have failed to develop a relatively simple and cost-effective method for forming discrete connective elements, such as pillars or bumps, on a die, and mating recesses on a substrate. Furthermore, even the most simple methods in the art fail to yield both a self-aligning feature and a dense array of sufficiently slender connective elements.